Up to the present time, deployment of fuel cells in field operations and military applications has been inhibited, due to an inability to use widely-available logistic fuels as a primary energy source. The term “logistic fuel” refers to any fuel that is approved by the U.S. government as acceptable for logistic maneuvers or field operations. Logistic fuels typically comprise a high concentration of sulfur. A preferred logistic fuel is a high sulfur-containing jet propulsion fuel, particularly JP-8 fuel, wherein the sulfur content specification ranges up to about 3,000 ppmw. Typically, the sulfur is present in the form of high molecular weight organosulfur compounds as identified, for example, by I. C. Lee and H. C. Ubanyionwu, in “Determination of Sulfur Contaminants in Military Jet Fuels,” Fuel, 87 (2008), pp. 312-318.
Reforming catalysts are severely affected by high sulfur-containing fuels, resulting in lower catalyst performance and endurance as evidenced by lower fuel conversion, reduced hydrogen output, lower thermal efficiency, and a higher concentration of coke precursors, as compared with sulfur-free fuels. For the purposes of this invention, a “coke precursor” is defined as an organic compound that tends under reaction conditions to form carbonaceous or graphitic deposits. Coke precursors include, for example, C2 compounds, such as ethane and ethylene, and C3 compounds, such as propane and propylene. A low quality reformate stream with an unacceptable concentration of coke precursors leads to carbon formation and increased pressure drop, and thus a failure of downstream fuel cell stacks. Moreover, at sulfur concentrations on the order of 3,000 ppmw as found in preferred logistic fuels, such as JP-8, the reforming catalyst tends to lose activity to an unacceptable level. Operating a fuel reformer at an elevated temperature may compensate for reduced catalytic activity; however, a higher temperature may hasten catalyst degradation.
To be suitable for use in fuel cell applications, high sulfur-containing fuels must be reformed in the presence of a reforming catalyst into a clean gaseous reformate, which preferably comprises a mixture of hydrogen and carbon monoxide essentially free of sulfur. As used herein, the terms “essentially free of sulfur” and “essentially sulfur-free” refer to a sulfur concentration of less than about 10 parts per million by volume (10 ppmv), preferably, less than about 5 ppmv, and more preferably, less than about 1 ppmv. Different approaches have been used to achieve acceptable fuel reforming starting from a high-sulfur containing fuel. Examples of five such approaches include: (a) plasma reforming, (b) oxidative desulfurization, (c) hydrodesulfurization, (d) pervaporation, and (e) liquid fuel desulfurizer sorption.
In the case of plasma reforming wherein no catalyst is used to assist in the reforming, the process is known to be sulfur tolerant. The main disadvantages, however, include an unacceptably large apparatus size, an unacceptably high parasitic power input, and unacceptably high electrode erosion at elevated pressures thus reducing longevity. As a reference, see L. Bromberg, D. R. Cohn, A. Rabinovich, and N. Alexeev, “Plasma Catalytic Reforming of Methane,” International Journal of Hydrogen Energy, 24 (1999), pp. 1131-1137.
As described for example in WO-A2-2008/025002 and disclosed by J. T. Sampanthar, et al. in “A Novel Oxidative Desulfurization Process to Remove Refractory Sulfur Compounds from Diesel Fuel,” Applied Catalysis B: Environmental 63 (2006), pp. 85-93, oxidative desulfurization may be effective in converting recalcitrant thiophenes and other refractory sulfur compounds into more tolerable polar sulfoxides, polar sulfones and other polar oxidation products; however, the process also has several disadvantages. Downstream multi-stage cleanup (extraction or adsorption) is needed to remove the polar sulfoxides or sulfones, followed by waste management of such extracted sulfur compounds. Moreover, employing an oxidant, such as hydrogen peroxide or pure oxygen, poses safety risks in portable or field operations. The overall system is expensive as well as bulky and, therefore, unacceptable for logistic operations. Moreover, the operational cost of an oxidative desulfurization system increases with increasing sulfur content in the fuel, due to stoichiometric or higher consumption of required oxidant relative to sulfur.
Hydrodesulfurization (HDS), as disclosed for example in U.S. Pat. No. 6,984,372B2, while being mature for large scale operations and acceptably efficient in removing thiols, sulfides, and disulfides, is far less effective for removing thiophenes, common species of which include dibenzothiophene and its alkylated derivatives. These compounds, especially 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene, render deep desulfurization by HDS unacceptably difficult. Moreover, catalytic activity towards removing alkylated dibenzothiophenes is further decreased in the presence of inhibitors like aromatics and organic nitrogen species, which are ubiquitous in fuels. Additionally, HDS requires a hydrogen feed, which implies a complex and large system.
The pervaporation approach, as described for example by Y. Wang, J. Latz, R. Dahl, J. Pasel, and R. Peters in “Liquid phase desulfurization of jet fuel by a combined pervaporation and adsorption process,” Fuel Processing Technology, 90 (2009), pp. 458-464, requires a permeation membrane to separate selectively the organosulfur compounds from the hydrocarbon compounds prior to reforming. Another disclosure is found by Ligang Lin, Yuzhong Zhang, and Ying Kong, in “Recent Advances in Sulfur Removal from Gasoline by Pervaporation,” Fuel, 88 (2009), pp. 1799-1809. The system, which takes a multi-stage approach, also involves challenges in handling the separated fuel fraction (typically 30%) containing the organosulfur compounds. Moreover, a low flux is achieved that makes the system bulky; and an energy intensive and expensive refrigeration system is generally required to condense light hydrocarbons at high vacuum. Finally, increasing aromatics, alkenes, and sulfur content in the feed may lead to increased flux and decreased organosulfur separation.
Liquid fuel desulfurization involving a sorbent-based adsorption unit may provide an improved methodology for developing integrated fuel processors that can operate with up to 3500 ppmw sulfur-containing fuels to produce fuel cell quality reformate. Efforts are being made to develop new adsorbents to remove thiophenic compounds from logistic fuels either via n-complexation, van der Waals and electrostatic interactions, or via reactive adsorption by chemisorption at elevated temperatures. Despite considerable efforts put into regenerable liquid fuel desulfurization via adsorption or fuel fractionation/organosulfur compound adsorption, the best approaches have resulted in systems of considerable size and complexity due to their low sulfur sorption capacity. Also, the process removes disadvantageously about 5 percent or more, by weight, of the fuel and has associated parasitic losses for equipment operation (e.g., high pressure drop across the sorbent bed) due to problems associated with organosulfur adsorption. Many commercially available sorbents have unacceptably low sulfur adsorption capacity, low adsorption duration (i.e., fast breakthrough time), and high liquid fuel retention. Moreover, regeneration of the sorbent bed requires good thermal management in order to avoid temperature excursions and hot spots that can rapidly deactivate the sorbent materials. As an example of liquid fuel desulfurization, see U.S. Pat. No. 7,063,732B2, US 2007/0092766A1, and US 2004/0118747A1.
In summary, none of the approaches discussed hereinabove is sufficiently simple, compact, economical, and versatile to allow for reforming of a wide range of high sulfur-containing liquid fuels over an acceptable period of operation. Thus, the clean reformate that is needed as a primary source of energy, especially as may be required in logistics and field operations and in advanced fuel cell stacks, remains far from reach.